Conventionally, when producing a semiconductor memory such as a DRAM, a redundant circuit configuration has been employed to prevent yield of DRAM production from being lowered due to random defects in the production process. Namely, for example, a large number of memory cells are integrally formed in a chip of, for example, a DRAM, to increase the memory capacity. Even if the chip includes only one defective memory cell, the chip is rejected and hence the production yield is decreased.
When a failure occurs in a memory cell with the redundant circuit configuration, the defect is recovered, for example, as follows. A word line or a bit line which is used to specify an address (via an address decoder) and which is connected to the defective memory cell is disconnected from the address decoder. Then a word line or a bit line connected to a spare or backup memory cell is connected to the address decoder. As a result, the failed memory cell is replaced with a reserve memory cell and hence the production yield is improved.
As a practical method of replacing the defective memory cell with a spare memory cell, there has been commonly adopted a laser fuse linking procedure. In this method, for example, a fuse is disconnected by a laser beam spot to replace the failed memory cell with a spare memory cell.
Also broadly employed is an electric fusing scheme in which the fuse is electrically melted down for the above purpose, which has been described in, for example, the JP-A-60-65545, JP-A-62-139198, and JP-A-4-14700.
However, the laser fusing method is attended with a disadvantage of a relatively complex process in which a target position of a laser beam is controlled according to information from a circuit tester. Moreover, the apparatus to illuminate a laser beam spot onto a target position is required to control the target position with a high precision. In addition, damage to peripheral circuits occurs when disconnecting the fuse. Similary, the electric fusing method is also attended with disadvantageous features of adverse influence upon peripheral circuits and damage to a substrate of the memory when the fuse is electrically melted down.